[Openstreetmap-dev] Re: [Openstreetmap] Landsat update

Jo Walsh jo at frot.org
Sat Apr 30 15:17:51 BST 2005


re nick,
On Sat, Apr 30, 2005 at 01:03:00PM +0100, Nick Whitelegg wrote:
> I'm wondering if it's to do with the inaccuracy of WGS-84 at the UK's latitude

no, apart from the usual GPS error factor it shouldn't make a difference.
 
> i.e. it uses Airy coordinates even though the GPS uses WGS84.
> to screen display, the error becomes worse!

http://www.free-map.org.uk/drawmap.php?e=490806&n=130314&mapview=10 
what's being passed in here is OSGB projected numbers, yes? 
i now feel quite confused as to what you're reprojecting where :)

> I think it must be a projection issue rather than inaccuracy in my GPS, 
> because the SRTM height data and my GPS tracks correlate very well. 

agreement with this. quick consultation with Schuyler, couple of
suggestions as to what the problem is:

- with SRTM at 90M and Landsat at 15 or 30, it's possible that the
  appearance of accuracy over the SRTM layer is a lucky coincidence?
  or do your other experiences bely that?

- the projection error could well be on the landsat WMS side;
  "misregistered" or slightly misrectified tiles. Can you try this out
  against a different set of known good data and check the fit?

happy to try to help thrash this out on irc...


zx





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